Indolenine cyanine dyes which are excellent in heat resistance and water resistance are advantageously used as optical data recording media constituting recording layers of various optical recording disks (see, for example, Japanese Unexamined Patent Publication No. 59-24692 (1984)). However, the indolenine cyanine dyes are liable to cause data reproduction deterioration due to repeated light irradiation for data reproduction and to cause photo-deterioration during storage in light and, hence, it is difficult to keep a recording layer employing such an indolenine cyanine dye stable during a prolonged use. Therefore, where an indolenine cyanine dye is used to form a recording layer, a metal complex capable of functioning as a singlet oxygen quencher is mixed with the indolenine cyanine dye, and a coating solution prepared by dissolving the mixture in a solvent is applied onto a resin substrate of an optical recording disk for the formation of the recording layer (see, for example, Japanese Unexamined Patent Publication No. 59-55794 (1984)).
If such a metal complex is capable of functioning not only as a singlet oxygen quencher but also as an optical data recording medium per se, the metal complex would be expected to have an enhanced usefulness. For example, a recording layer of an optical recording disk can be formed of the metal complex alone without the use of the indolenine cyanine dye.
It is therefore an object of the present invention to provide a novel metal complex which is capable of functioning not only as a singlet oxygen quencher but also as an optical data recording medium per se.